The present invention relates to an optical module for use in an optical communication system.
Recently, development of an optical module using a PLC (Planar Lightwave Circuit), which has a laser diode (LD), a photo diode (PD) and electric elements integrated, has progressed in an optical communication system. For example, in the Electronics Society Meeting C-3-6, 1997, of the Electronic Information Communication Society and in JP-A-191152/1997 were proposed an optical module having a light-emitting element (the LD and so forth) and a photoreceptive element (the PD and so forth) mounted on a waveguide substrate.
FIG. 10 is a perspective view illustrating a schematic configuration of the optical module disclosed in the above-mentioned documents. This optical module is a module having a light-emitting element 102 and photoreceptive elements 108 and 109 mounted on a waveguide substrate 101 in which waveguides 104 to 107 were formed. The waveguides 104 to 107 configure the waveguide having a symmetrical Y-branch structure as a whole. Namely, this symmetrical Y-branch waveguide includes one single-mode waveguide section (waveguide 104), a taper section (waveguide 105) in which the waveguide spreads about two times, and a branch waveguide section (waveguides 106 and 107). The end face of the waveguide 104 is optically coupled to the end face of an optical fiber which introduces the send light to the exterior or into which the receive light from the exterior is introduced. The end face of the waveguide 106 is optically coupled to one side's light-emitting surface of the light-emitting element 102, and the end face of the waveguide 107 is optically coupled to the photoreceptive surface of the photoreceptive element 109. The photoreceptive element 108, which is a photoreceptive element for a send monitor, is provided so that the photoreceptive surface thereof faces the other-side's light-emitting surface (backside) of the light-emitting element 102.
In this optical module, light emitted from one side's light-emitting surface of the light-emitting element 102 is propagated through the waveguides 106, 105 and 104 sequentially and is output, and the above output light is propagated to the exterior via the optical fiber. On the other hand, input light from the optical fiber is propagated sequentially through the waveguides 104, 105 and 107 to enter the photoreceptive surface of the photoreceptive element 109, and is converted into an electric signal. Light emitted from the backside of the light-emitting element 102 is received in the photoreceptive element 108 for the send monitor.
By connecting the optical modules such as described above via the optical fiber, bi-directional communication becomes possible.
In the optical module, high optical output power, high photoreceptive sensitivity and cost reduction are indispensable, and as one of the important tasks for accomplishing these are cited highly efficient optical couplings between the optical active element such as the light-emitting element and the photoreceptive element, and the waveguide, and between the optical fiber and the waveguide.
In general, as to the optical fiber and the optical waveguide, a photoreceptive angle at which incident light is guided has been determined, and light that entered beyond its photoreceptive angle is not guided, resulting in an optical loss. For example, with the optical fiber, the photoreceptive angle is up to 5° in a single mode system, and up to 12° in a multi-mode system. For this reason, for example, so as to carry out a highly efficient coupling between the light-emitting element and the waveguide, it is necessary to cause the send light from the light-emitting element to efficiently enter the waveguide within a determined photoreceptive angle to restrain the optical coupling loss at a low level. But, since the conventional optical module described above did not include such a configuration, room was still left for improving in this point.
Additionally, it was proposed that, by using a micro optical system such as a lens and a tip-ball fiber (one having the tip section of the optical fiber processed in a sphere) to convert a large angle of divergence of the LD into a small photoreceptive angle of the optical waveguide, optical coupling efficiency between the LD and the waveguide was enhanced; however, in this case, parts such as the lens increase, and simultaneously, assembling process increases, whereby the problem occurs in terms of the cost reduction.